1. Field of the Invention
The present invention is directed to methods and apparatus for use with subterranean drilling systems. More specifically, the present invention is directed to systems to maintain a constant and desired weight on a drilling stem to maximize penetration and drilling rates.
2. Description of the Prior Art
In earth drilling, particularly the drilling of oil and gas wells, the control of the drilling operation has usually been accomplished manually. Conventional drilling rigs utilize a draw works which is powered by an engine and operates most of the motor driven portions of the rig. The draw works includes a drum with a drill line wound on it which is fed off to lower drill pipe as the drilling is accomplished. The drill line is looped through a crown block in a double pulley relationship and the end of the line is connected to a fixed point end called the dead line.
As the pipe is lowered into the well during drilling, the weight of the pipe string on the drill bit is measured by the tension in the drill line. The tension in the drill line is commonly measured by a pressure sensor which converts tension to weight indication through a hydraulic line extending to a bit weight gauge on the drilling console. The rate of feed out of the drill line from the drum controls the bit weight and to a large extent the rate of drilling. The rate of feed out of the drill line from the drum is controlled by a hand brake operated by a conventional brake lever. In manually-operated drilling rigs, the driller has to monitor the operation of the equipment and operate the brake from time to time in response to the indications of the bit weight gauge to control the rate of feed out of the drill line and thus attempt to keep a fairly constant bit weight.
In recent years, there have been developed a number of automatic drilling systems. These systems are automatic in the sense that they provide some form of automatic control over the equipment. Many of these automatic drillers operate from the air supply of the drilling rig en route to the drillers control station. The components involved are mainly air clutches with various types of air dump valves to exhaust used air.
One such device is disclosed in U.S. Pat. No. 4,491,186 (“the '186 patent”) as issued to Adler. The apparatus disclosed in the '186 patent utilizes the rotation rate of a downhole mud motor as a parameter to determine the release of the drill string. While this invention has application to downhole mud motors, it requires the use of an in-hole tachometer which enhances operation cost.
Another automatic drilling system is disclosed in U.S. Pat. No. 5,474,142 (“the '142 patent”) as issued to Bowden. The device illustrated in the '142 patent operates off of bit weight and fluid pressure which act on a pair of Bourden tubes. While such a device is nominally effective as an automatic driller, the use of the Bourdon tubes creates a time lag and limits its sensitivity due to the necessity for pressurization of hydraulic fluid.
Yet another system is illustrated in U.S. Pat. No. 5,713,422 as issued to Dhirdsa. This system suffers from the use of multiple sensing assemblies to determine and calculate the rate of penetration, which assemblies are maintenance intensive and are thus problematic for long term operation. Those in the drilling industry desperately need to have an automatic driller that would significantly improve the constancy of WOB, and make necessary changes with more smoothness (known as “peeling the drum”) than is possible with existing systems and technology. The use of emerging bit technology such as PDC bits especially require a smooth action to prevent the breaking of expensive diamond cutters.
Also, the industry needs technology and systems which allow considerably faster drilling than is now possible. Existing automatic driller systems cannot be used in many circumstances, because the rate of penetration (ROP)is deemed to be too fast for conventional technology to keep up.